JP2782071B2 - Optical rotator - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical rotator, and more particularly to an optical rotator capable of correcting a Faraday rotation angle variation due to a temperature variation of a Faraday rotation element.

(Prior art) The Faraday rotator has a characteristic that the optical rotation changes according to the strength of the magnetic field applied thereto, and this characteristic is used recently in a transformer for an instrument for a power meter, so-called optical CT. ing.

FIG. 6 is a diagram showing a schematic configuration of an optical CT using a Faraday rotator, in which a light emitted from a light source 1 is applied to a polarizer 2;
To obtain linearly polarized light. Here, for example, when linearly polarized light having an azimuth parallel to the X axis is irradiated on the next-stage Faraday rotator, the oscillation direction of the linearly polarized light exiting the Faraday rotator fluctuates according to the magnetic field applied thereto. The incident light is made incident on the polarizing beam splitter, and the X-axis component, Y
The light is separated into axial components, each of which is received by the light receivers 5 and 6, and how much the linearly polarized light is rotated, that is, how much the magnetic field is applied to the Faraday rotator can be determined from the received light amount.

However, the optical rotation power of the Faraday rotation element has a temperature dependence, so that even in a magnetic field of the same strength, the rotation angle varies with the change of the ambient temperature, causing a measurement error, and reducing the measurement error. In addition, it is necessary to select a material having a small change in the optical rotation angle due to a temperature change as the Faraday rotation element.

However, in general, a Faraday rotation element using such a material has a drawback that the Faraday rotation element has to be large in order to have a small Verde constant and obtain a desired optical rotation.

(Object of the Invention) The present invention has been made in view of the above-mentioned drawbacks,
Using a small Faraday rotator with a high Verde constant, the rate of change of the optical rotation angle with temperature can be set to an arbitrary value so that the detected value is not affected by temperature changes when configuring a small transformer, for example. It is an object of the present invention to provide a rotator that can be set to.

(Summary of the Invention) In order to achieve this object, an optical rotator of the present invention comprises a first phase plate that converts incident linearly polarized light into elliptically polarized light having a desired azimuth and ellipticity, and light transmitted through the first phase plate. Is combined with a second phase plate for converting the first phase plate into a linearly polarized light, and the first phase plate is rotated such that the amount of change in the rotation angle with respect to the temperature change of the first phase plate is twice the desired amount of optical rotation angle correction. The plate thickness is set, and the rate of change of the optical rotation angle with respect to the temperature is set arbitrarily.

(Examples) Hereinafter, the present invention will be described in detail based on examples shown in the drawings.

First, various characteristics of the Faraday rotator will be briefly described prior to the description of the present invention to facilitate understanding of the present invention.

For example, the optical rotation angle θ of the Faraday rotator in the state shown in FIG. Can be represented by

That is, the greater the Verdet constant, the number of equivalent interlinkage, and the current flowing through the conductor, the larger the optical rotation angle θ and the higher the detectability. Therefore, when the Faraday rotator is used as a current transformer for detecting a small current and being miniaturized, a large Verdet constant V is required. The Verdet constant is particularly large in a ferromagnetic material containing a rare earth element. However, since it has a temperature characteristic, the angle of rotation varies depending on the temperature, and it is difficult to perform a correct detection. It is.

FIG. 8 is a diagram showing an example of a change in the temperature-rotation degree of the Faraday rotation element. As shown in FIG. 8, the rotation angle at 25 ° C. when no magnetic field is applied is 45 deg.
If there is a change in the optical rotation angle of deg, the temperature-optical rotation characteristic Δθ / ΔT of the Faraday rotator is ΔθF / ΔT = (45deg−46.5de) g / (25 ° C.−90 ° C.) = ( −1.5 deg / −70 ° C. = 0.0214 deg / ° C. Therefore, an optical component having the following characteristics for correcting the temperature characteristics of the Faraday rotating element: ΔθC / ΔT = −0.0214 deg / ° C. May be arranged before or after the Faraday rotation element.

FIG. 1 is a diagram showing the principle of the present invention. In the figure, 11
Is a first phase plate whose optical axis is rotated by 45 degrees with respect to the incident linearly polarized light, and 12 is a second phase plate whose optical axis is parallel to the linearly polarized light incident on the first phase plate.

The linearly polarized light that has entered the first phase plate 11 produces a phase difference θ, and enters the next-stage second phase plate 12 to become linearly polarized light again. When displaying the status at the Poincare sphere as shown in Figure 2, previously Dzu S linearly polarized light in a point becomes 90deg rotated circularly polarized light about the S ₂ axis by entering the first phase plate, the next stage returns to linearly polarized light by 90deg rotates around the S ₁ axis in the second phase plate.

That is, an optical rotator may be formed by combining two phase plates, and it may have a temperature characteristic for correction.
Therefore the amount of change in the rotation angle of the S ₂ axis due to temperature changes is set to a desired value, it may be configured to polarization rotator that having a desired temperature characteristic is further 90deg rotated about the S ₁ axis.

Based on this method, the above-mentioned optical component for compensating the temperature characteristic of the Faraday rotator is constituted by the phase plate material, the optical axis azimuth, and the cutting angle so that a change in the optical rotation angle of 1.5 deg with a temperature change of 70 ° C. , Length, phase wavelength, and the like may be selected. Hereinafter, a method of setting various parameters using mathematical expressions will be described.

Here, for simplicity of description, a Y-cut quartz plate is used as a phase plate, the wavelength used is 780 nm, the incident angle of light is 0 deg, and the operating temperature range is 25 ° C. to 90 ° C. However, the circular birefringence characteristic of quartz is neglected.

Birefringence in the lens is _{n o (λ) = 1.53152 +} 4369 / λ 2 +1.378 · 10 4 / λ 4 = 1.53152 + 4369/780 2 +1378 · 10 4/780 4 = 1.538738 ......... n e (λ) = ^{1.54022 + 4505 / λ 2 +1.521 ·} 10 4 / λ 4 = 1.547666 is ........., also birefringence due to temperature changes is _{n o '(λ.T) = n} o (△ n o / △ T) _{△ T = n o + (-} 0.547 · 10 -5) △ T = 1.538738-0.547 · 10 -5 · △ T ......... n e '(λ.T) = n e + (- 0.651 · 10 -5) ΔT = 1.54766−0.651 · 10 ⁻⁵ · ΔT (where ΔT = T−25 ° C.)

On the other hand, the linear expansion coefficient 1 / l (△ l / △ T) of quartz is 1 / l (△ l / △ T) = (13 cosγ + 7 sin γ) · 10 ⁻⁶ / ° C. = (13 cos 0 + 7 sin 0) · 10 ⁻⁶ / ° C. = 13・ 10 ^-6 / ℃ ……… The angle Γ at which the polarization direction rotates when light is incident on the phase plate is Γ = 360 / λλlｌn ・ (1 + 1 / l (△ l / △ T) △ T) / cosγ ( Δn = n _e ′ + n _o ′)... Here, for example, assuming that the rotation angle Γ is 90 deg, the plate thickness 1 at 25 ° C. is obtained as 1 = 90 · 780 · 10 ⁻⁶ /360·8.928·10 ⁻³ = 0.0218366 mm... = 21.8366 μm. When the thickness of the quartz plate is used and the rotation angle?

That is, in a phase plate with a plate thickness of 21.8366 μm that gives a rotation angle of 90 degrees, the rotation angle changes by 0.65 degrees with respect to a temperature change of 70 ° C.
1.5 deg × 2 = 0.65 × a = 3 deg / 0.65 = 4.61 To correct the fluctuation of the rotation angle based on the temperature change of the Faraday rotation element as described above, it is 4.61 times the plate thickness as is clear from the equation. What is necessary is just to select a phase plate. Therefore, the selected thickness of the phase plate is 21.8366 × 4.6154 = 100.784 μm.

Table 2 shows the rotation angle に 対 す る with respect to the temperature change when a phase plate having a thickness of 100.784 μm is used, using Equation 8, and as is clear from the table, the rotation angle に 対 し with respect to the temperature change of 70 ° C. The angle 3.0 changes by 3.01deg.

If the phase plate having a thickness of 100.784 μm is the first phase plate, and the phase plate having the thickness of 21.8366 μm is the second phase plate, and the configuration shown in FIG. 1 is adopted, the characteristics are as shown in FIG. It will be a kimono.

FIG. 5A is a diagram showing the characteristics at 25 ° C. on a Poincare sphere, in which the linearly polarized light at the point S enters the first phase plate, rotates 415.38 degrees around the S ₂ axis, and rotates at the point P. Moving. Then 90deg about the S ₁ axis when entering the second phase plate
Due to rotation, it becomes linearly polarized light having an inclination of 27.69 deg. When the temperature changes by 70 ° C., as shown in FIG.
41 about the S ₂ axis incident on the first phase plate linearly polarized light in a point
2.37 ° rotation and move to P '. Next substantially linearly polarized light having an inclination of 26.19deg for 89.35deg rotates around the S ₁ axis when incident on the second phase plate.

That is, as shown in FIG. 4, the overall characteristics of the first and second phase plates are such that the light at point S is linearly polarized light having a gradient of 27.69 deg at 25 ° C. and has a gradient of 26.19 deg at 95 ° C. It is made to be almost linearly polarized light and has a function as a so-called optical rotator.

When the first and second phase plates having the above-mentioned structure are configured as shown in FIG. 5 to correct the temperature characteristics of the Faraday rotator having the above-described characteristics, the first and second phase plates are accompanied by a temperature change occurring in the Faraday rotator. It can be seen that the change in the optical rotation is opposite to the change in the rotation angle due to the temperature change occurring in the first and second phase plates, and the rotation is canceled out, so that the temperature-optical rotation characteristic of the entire optical system is constant.

That is, the temperature change of the first phase plate {the rotation angle change amount at T} is equal to the optical rotation change degree ΔθF generated by the Faraday rotation element.
The thickness 1 of the first phase plate may be set so as to be twice the thickness of the first phase plate.

In the embodiment of the present invention, the optical rotation angle at 25 ° C. of the optical rotator constituted by combining two phase plates is set to 27.6.
Although it was set to 9 deg, the present invention is not limited to this, and a desired optical rotation can be obtained by appropriately selecting the incident angle of light and the cutting angle of the phase plate, for example, when a magnetic field is not applied at 25 ° C. The output of the Faraday rotator is 45deg × n (n =
If the whole optical system is adjusted so that the polarization direction is (integer)
When splitting the output with PBS, the light intensity of each spectral output is 1: 1
Alternatively, since the ratio is 1: 0, it is possible to configure a current transformer that has high detection accuracy and can easily measure.

As described above, in order to set the output of the Faraday rotator at 25 ° C. and in the absence of a magnetic field to linearly polarized light having a polarization direction of 45 × n, the rotation angle に お け る in the first phase plate is given by Γ = nx180 + 90. The thickness 1 may be selected so that the rotation angle variation △ Γ due to the temperature change becomes a desired value. For example, in this embodiment, the plate thickness is set so that the rotation angle に お け る of the first phase plate becomes 4 degrees.
If 109.183 μm is selected, as shown in Table 3, 4
It is 446.74deg at 50deg and 95degC, and it can be composed of a rotator having a change of 1.63deg with respect to the temperature change of 70degC, and the temperature-rotation characteristic of the Faraday rotator can be made almost constant. .

In the embodiment of the present invention, the two phase plates have been described as being independent. However, the present invention is not limited to this.

(Effects of the Invention) Since the present invention is configured and functions as described above, it is possible to arbitrarily set the amount of change in optical rotation with temperature change. For example, the present invention can be used as a correction optical component such as a Faraday rotation element. By using the optical rotator according to the present invention, a small current transformer can be configured without being affected by a temperature change.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of the present invention, FIG. 2 is a diagram schematically illustrating the principle of the present invention on a Poincare sphere, and FIGS. 3 and 4 are diagrams illustrating the function of the optical rotator of the present invention on a Poincare sphere. FIG. 5 is a diagram schematically shown above, and FIG. 5 is a diagram showing an embodiment in which the optical rotator of the present invention is used for correcting the temperature characteristics of the Faraday rotator.
FIG. 1 is a diagram showing an embodiment of a current transformer using a conventional Faraday rotator, FIG. 7 is a diagram for explaining characteristics of the Faraday rotator, and FIG. 8 is a temperature-rotation degree of the Faraday rotator. It is a figure showing an example of a characteristic. DESCRIPTION OF SYMBOLS 1 ... Light source, 2 ... Polarizer, 3 ... 7 Faraday rotation element, 4 ... Polarization beam splitter, 5, 6 ... Receiver, 11 ...
... First phase plate, 12... Second phase plate.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G02B 27/28 G02B 5/30 G01R 15/22

Claims (1)

(57) [Claims] An optical rotator disposed before or after a Faraday rotator, comprising: a first phase plate for converting incident linearly polarized light into elliptically polarized light having a desired azimuth and ellipticity; and a first phase plate. The first and second phase plates are combined with a second phase plate that converts the transmitted light into linearly polarized light, and the first and second phase plates are rotated so as to cancel a change in the optical rotation angle due to a temperature change of the Faraday rotator.
The optical rotator wherein the thickness of the phase plate is set.